In systems of the above noted type, the electron beam impinges onto the work piece along a predetermined processing path. Additionally, the electron beam is deflected relative to the processing path in accordance with modulation signals superimposed on the main deflection signals.
The term "work piece" as used herein is to be understood to mean two separate parts which make contact along an interface and which are to be welded to one another along the interface.
In conventional systems of this type, the processing path as well as the modulation curve are approximated by analytic functions generated by function generators. These function generators generate only analytic curves such as straight lines, circles, parabolas, etc. Superimposition of these curves then cause the electron beam to be deflected along a zigzag or a spiral path. Additionally, use of the function generators requires that the beam be deflected along its path with a specified velocity. This velocity depends upon the curve to be generated and cannot be adjusted.
Impingement of the electron beam onto the work piece causes its energy to be transferred into the work piece in the form of heat. The material is heated locally above the melting and vaporization temperature. A liquid zone is thus created which has an interface with solid material. Further, a capillary tube of vaporized material is created within the liquid zone, directly at the point of impingement of the electron beam. The interface between the vapor capillary and the liquid zone is determined by the energy balance requirements in the boundary region. During the advance of the work piece, the material in the vicinity of the front surface of the vapor capillary is melted, while that in the region of the trailing or rear surface hardens. The melted material must thus move from in front of the vapor capillary, around the latter, into the region of the rear surface. Measurements indicate that only a very small percentage (less than 2%) of the melt is vaporized in a groove welding process. By far the greatest part flows around the capillary with a speed in the order of magnitude of the speed of advancement of the work piece. The melting and hardening process therefore is subject to the laws of fluid mechanics.
Because of the pressure differences which are created during the welding process between the inside of the vapor capillary and the welding chamber, a vapor flow from the capillary into the chamber is created which can create dynamic effects. A corresponding effect can take place on the lower side of a work piece, if, during groove welding, an abrupt decrease of pressure takes place in the capillary. Such a decrease in pressure tends to cause the rear surface of the capillary to enter into the capillary interior and become directly exposed to the electron beam. This material is then vaporized by the electron beam, causing a pressure increase and a further movement of the phase interface. This in turn causes the formation of spikes, that is the formation of irregular sharp piping which decreases the fatigue strength of the work piece. For groove welds, an irregular formation of the welding bead results, in conjunction with a large spatter formation. The stronger the capillary boundary under these dynamic conditions, the higher the pressure differences, and the greater the spikes.
These effects take place in particular if the electron beam is not caused to oscillate around the processing path. Whether more lobe-shaped or trumpet-shaped capillaries are formed depends upon the focussing of the beam. The lobe-shaped capillaries with a narrow neck tend to create spatter on the side of the upper bead, while the trumpet-shaped capillaries with a pointed base tend to create them on the lower side, if groove welding takes place in the downward direction.
Oscillation of the electron beam along a modulation curve relative to the processing path causes the cross section of the capillary to increase and to be more uniform throughout its length. Thorough investigations of electron beam welding processes have shown that the structure of a weld is greatly influenced by micro-movement of the electron beam during the welding process; that is the quality of the weld varies considerably with changes in the shape of the modulation curve, the velocity along the processing path, and the velocity of the beam along the modulation curve. The optimum modulation for an individual case can generally only be determined empirically, taking into consideration the material to be processed and the geometric dimensions in the region of the gap to be welded. With the presently available analytic function generators, such an optimization can be carried out only within very narrow bounds.